Construction and Regulation of a Nanometer-Femtosecond-Scale Spatiotemporally Localized Chiroptical Source in a Single Plasmonic Nanoantenna.

Achieving precise control over chiral light-matter interactions at the nanometer-femtosecond scale, where both spatial and temporal limits are approached, remains a central challenge in nano- and quantum optics. This control is essential for next-generation ultracompact, ultrafast chiral photonic devices, but no method has realized both the construction and the active regulation of a localized chiroptical source within a single plasmonic nanoantenna. In this study, we report a strategy for creating and dynamically controlling a spatiotemporally localized chiroptical source via plasmonic eigenmode engineering in an achiral rectangular nanoantenna. With time-resolved photoemission electron microscopy, we directly image and analyze eigenmode dynamics and interference-induced near-field chirality across the space, time, and wavelength domains, revealing polarization-dependent hotspots in the nanoantenna. An interferometric pump-probe method further enables the on/off switching and near-field chirality reversal of the chiroptical photon source between nanoantenna corners with a sub-1.37 fs time delay between pump and probe pulses. This approach also yields a tunable superchiral photon source, providing a versatile platform for integrated ultrafast chiral nanophotonic applications.